The present invention relates to a temperature sensing system, and more particularly to an interferometric temperature sensing system that uses a laser detection element to measure a temperature by detecting changes in an interference fringe pattern indicative of changes in external temperature.
A thermocouple or resistive temperature detector (i.e., RTD) is typically used for general methods of electrically measuring the temperature of heat-emitting objects, objects within a hot environment or an ambient temperature of a hot (i.e., beyond room temperature) environment. Temperature measuring meters using such above methods have problems however, with electromagnetic interference (EMI), accuracy, response speed, and resolution. Also, there is a problem in that high temperatures are not easily measured. To solve many of the problems associated with the above mentioned temperature measuring means, an interferometric temperature measuring method has been developed.
The interferometric temperature measuring method uses an interferometric sensor formed on the end of an optical fiber. Temperature is measured by detecting variances in an interference fringe pattern between two laser beams within the sensor having different paths. Such an interference temperature measuring method is disclosed in Optics Letter, Vol. 17, No. 14, pp. 1021 in an article entitled SAPPHIRE FIBER BASED INTRINSIC FABRY-PEROT INTERFEROMETER by Ando Wang. Another interference temperature measuring method is disclosed in U.S. Pat. No. 4,714,342 filed on Dec. 22, 1987 by David Jackson. These references show that a change in an interferometric fringe pattern generated within the interferometric sensor can be detected by using electronic equipment including a single-mode fiber directional coupler and a photo diode. Once the degree of change in the interferometric fringe pattern is determined, a corresponding temperature can be calculated.
In contemporary practice, a laser beam emitted from a laser generation source is transmitted to one side of the single-mode fiber directional coupler and reflected back onto the other side. The light is then detected in a photo diode connected to the opposite side of the single-mode fiber directional coupler. The light detected by the photo diode is converted into a corresponding temperature by an electronic circuit. These techniques typically use items such as a single-mode optical fiber, a single-mode fiber directional coupler, a laser diode, an optical detection element, or a temperature conversion circuit, among other circuit components.
An optical temperature sensing system composed of these components may detect an amount of change in interference light generated within the sensor, i.e. the change in the interference fringe pattern as described above by using the photo diode is converted into a corresponding temperature using an electronic circuit.
Contemporary available devices often use interferometers having different structures. The optical temperature sensor introduced by Wang, et al. uses an interferometric sensor having a Fabry-Perot structure. The Fabry-Perot interferometric sensor produces variations in phase difference between two laser beams having different reflecting paths, corresponding to the temperature. On the other hand, the interferometric temperature sensor disclosed in U.S. Pat. No. 4,714,342 issued to Jackson uses an interferometric sensor having a Michelson structure. That is, Jackson's sensor varies the phase difference between two laser beams respectively reflected onto a signal optical fiber and a reference optical fiber, according to the temperature.
Both of the devices described above transmit interfering light from an interferometric sensor to a photo diode using a single-mode fiber directional coupler. In response, the photo diode converts a sensed signal into an electrical signal that is signal processed to determine a measured temperature. Implementation of such conventional interferometric temperature sensing methods is not easy. Such methods are plagued by many difficult problems. Also, since expensive optical elements such as a single-mode fiber directional coupler is required for implementation, practical use of such devices and methods is limited.
One more recent effort to create an interferometric device is disclosed in U.S. Pat. No. 5,202,939 entitled Fabry-Perot Optical Sensing Device For Measuring A Physical Parameter issued to Belleville et al. on 13 Apr. 1993. This device uses a Fabry-Perot interferometer through which a light signal is passed, an optical focusing device for focusing at least a portion of the light signal outgoing from the Fabry-Perot interferometer, and a Fizeau interferometer through which the focused light is passed. A multimode optical fiber optically couples the Fabry-Perot interferometer with a light source. Although this effort purports to achieve satisfactory results during operation, we find that many of the aforementioned problems regarding cost and ease of implementation are still present.